Optically pumpable waveguide amplifier with amplifier having tapered input and output

ABSTRACT

Optically pumpable waveguide amplifier with amplifier having tapered input and output. The present invention provides for a optically pumpable waveguide amplification device that includes: a cladding material; a passive optical waveguide embedded in the cladding material that has no optical amplification functionality; and an active optical waveguide having an input portion, a middle portion and an output portion, including: a gain material with a higher refractive index than the passive optical waveguide, wherein the middle portion of the active optical waveguide is embedded in the cladding material, and faces the passive wave guide, such that a lower surface of the middle portion is an upper surface of the passive optical waveguide. There is also provided a device for optically pumpable waveguide amplification and a method for signal radiation amplification.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from PCT Application PCT/IB2014/063117under 35 U.S.C. §371, filed on Jul. 15, 2014, which claims priority fromUnited Kingdom Application No. 1313282.4, filed Jul. 25, 2013. Theentire contents of both applications are incorporated herein byreference.

TECHNICAL FIELD OF THE INVENTION

The invention relates in general to the field of optically pumpedwaveguide amplifier devices and related devices and methods.

BACKGROUND OF THE INVENTION

Optical amplifiers compensate for the losses in optical communicationchannels by increasing the signal power. Erbium-doped fiber amplifiers(EDFA) are important to optical communications and have enabledtransmission of large amounts of data over long distances. Planaroptical waveguide amplifiers are expected to reach widespread usage inthe future as a result of the increased utilization of integratedphotonics. In particular, polymer waveguide technology is expected toincrease chip/board level communication capacity at low assembly costand will have a profound impact on the growing field of siliconphotonics. However, due to the restricted power budget, amplification ofthe optical signal can be required. Polymer waveguide amplifiers usuallymake use of optical pumping for operation. These amplifiers operate witha pump light, which have a shorter wavelength than the signal light. Theabsorption of the pump light leads to population inversion, andconsequently to amplification by stimulated emission.

Along with several other important characteristics, an idealoptically-pumped waveguide amplifier should include some keycharacteristics. First, the optical signal should have large overlapwith the gain material in the active waveguide for an efficientamplification. The modal gain is proportional to this overlap value.Second, the overlap integral between the signal and the pump modesshould be large to make use of population inversion efficiently. Third,outside the amplifier, the pump has to be separated from the signalefficiently. Also, the amplifier should preferably not deteriorate theoptical characteristic of the passive waveguide (e.g. propagation lossshould not increase). Last, it is preferable to have high tolerance tovariations of the device geometry and material characteristics comparedto the nominal design specs. This reduces the cost by increasing theyield and relaxing process parameters.

State-of-the-art optically pumped waveguide amplifiers can satisfy onlysome of these requirements. The coupling and separation of the pump andthe signal is obtained by using devices, such as interferometers ordirectional couplers, which do not have high tolerance because of theirresonant operating conditions. Most of them make use of rare earth-dopedpolymers as gain material. This imposes stringent boundary conditions ondesign due to their narrow absorption band and the limited concentrationcaused by the tendency of these materials to form aggregates.

The prior art has important limitations. For example, U.S. Pat. No.6,549,688 describes an optical amplifier design, which makes use ofasymmetric Mach-Zehnder interferometers to multiplex and demultiplex thepump and the signal. Adiabatic couplers are proposed for couplingbetween different types of waveguides. These couplers do not separatethe pump from the signal. Also, US 2004/0081415 describes a planaroptical waveguide amplifier, in which the signal couples between theactive and passive waveguides with the help of a directional coupler.U.S. Pat. No. 5,381,262 describes an optical waveguide amplifier, whichincludes partial erbium doping in the waveguide where the signalpropagates. The pump light is coupled to the waveguide amplifier withthe help of a directional coupler. In addition, EP 0,561,672 describes awaveguide amplifier, in which a gain region is obtained by doping andthe pump signal is coupled in and out of the amplifier using directionalcouplers. Owing to a precise directional coupler design, coupling lengthfor the signal wavelength is half of that for the pump wavelength.Therefore, the signal remains on a same waveguide, whereas the pumpsignal switches between waveguides. EP 1,030,413 describes arare-earth-doped planar waveguide positioned on top of a passivewaveguide. The rare-earth-doped waveguide is tapered, perpendicularly tothe planar direction, to couple the pump and the signal between thewaveguides. The coupler does not separate the pump from the signal. Anexternal component is provided to separate the pump.

SUMMARY OF THE INVENTION

The present invention provides an optically pumpable waveguideamplification device. The device includes: a cladding material; apassive optical waveguide embedded in the cladding material that has nooptical amplification functionality; and an active optical waveguidehaving a gain material with a higher refractive index than the passiveoptical waveguide, and which successively includes: an input portion, amiddle portion, and an output portion. The middle portion successivelyincludes: a first taper portion, an amplifier portion, and a secondtaper portion, wherein the middle portion is embedded in the claddingmaterial and faces the passive waveguide, such that a lower surface ofthe middle portion is an upper surface of the passive optical waveguide.Also, each of the taper portions widens towards the amplifier portion,parallel to the lower surface, such that a narrow end of each of thetaper portions have a cross-sectional area that is smaller than across-sectional area of the passive optical waveguide at the same levelof narrow end.

Furthermore, the present invention provides a method for signalradiation amplification. The method includes the steps of: coupling asignal to be amplified from the passive optical waveguide to the activeoptical waveguide of an optical waveguide amplification device at afirst tape portion of the device through adiabatic mode transformation;allowing a pump signal and the signal to be amplified to propagate inthe amplifier portion of the optical waveguide amplification device; andcoupling the amplified signal back to the passive optical waveguide atthe level of a second taper portion of the optical waveguideamplification device.

The present invention further provides for an optically pumpablewaveguide amplification device. The device includes: a claddingmaterial; a passive optical waveguide embedded in the cladding materialthat has no optical amplification functionality; and an active opticalwaveguide having an input portion, a middle portion and an outputportion, including: a gain material with a higher refractive index thanthe passive optical waveguide, wherein the middle portion of the activeoptical waveguide is embedded in the cladding material, and faces thepassive wave guide, such that a lower surface of the middle portion isan upper surface of the passive optical waveguide.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a top view of a simplified representation of an opticalwaveguide amplifier, according to embodiments;

FIG. 2 is a 2D cross-sectional view of a simplified representation ofthe optical waveguide amplifier device as it is before a taperedsection;

FIG. 3 is a 2D cross-sectional view of a simplified representation ofthe optical waveguide amplifier device after a tapered section;

FIG. 4 is a 2D cross-sectional view of the optical waveguide amplifierdevice as it is before a tapered section, where the passive waveguide isa rib waveguide;

FIG. 5 is a 2D cross-sectional view of the optical waveguide amplifierdevice after a tapered section, where the passive waveguide is a ribwaveguide;

FIG. 6 is a top view of a simplified representation of a variant to FIG.1, wherein the passive waveguide to include tapered sections, orientedopposite to the tapered sections of the active waveguide, according tostill other embodiments;

FIG. 7A shows a refractive index of the optical waveguide amplifierdevice.

FIG. 7B shows a mode profile at the inner side of a tapered section,illustrating how a signal is transferred between the active and passivewaveguides, whereas a pump signal remains in the active waveguide.

FIG. 7C shows a mode profile at the inner side of a tapered section,illustrating how a signal is transferred between the active and passivewaveguides, whereas a pump signal remains in the active waveguide.

FIG. 7D shows a refractive index of the optical waveguide amplifierdevice.

FIG. 7E shows a mode profile at the outer side of a tapered section,illustrating how a signal is transferred between the active and passivewaveguides, whereas a pump signal remains in the active waveguide.

FIG. 7F shows a mode profile at the inner side of a tapered section,illustrating how a signal is transferred between the active and passivewaveguides, whereas a pump signal remains in the active waveguide.

F) at the inner (left-hand column) and outer (right-hand column) sidesof a tapered section, illustrating how a signal is transferred betweenthe active and passive waveguides, whereas a pump signal remains in theactive waveguide;

FIG. 8 is a flowchart illustrating high-level steps of a method ofamplification, according to embodiments; and

FIG. 9 schematically illustrates the variation of the width of anon-linear taper portion along its longitudinal axis, according tospecific embodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMODIMENTS

Some preferred embodiments will be described in more detail withreference to the accompanying drawings, in which the preferableembodiments of the present invention have been illustrated. However, thepresent invention can be implemented in various manners, and thus shouldnot be construed to be limited to the embodiments disclosed herein. Onthe contrary, those embodiments are provided for the thorough andcomplete understanding of the present invention, and to completelyconvey the scope of the present invention to those skilled in the art.

Referring to FIGS. 1-6, the present invention is an optically pumpablewaveguide amplifier device 100. The device includes: cladding material10, passive optical waveguide 20, and active optical waveguide 30. Thewaveguides 20, 30 are at least partially embedded in cladding material10 and can be referred to as waveguide cores 20, 30 in the alternative.Passive waveguide 20 has no optical amplification functionality, whereasactive optical waveguide 30 includes: a gain material having a higherrefractive index than the passive optical waveguide.

The active waveguide successively includes: an input portion 31, amiddle portion 32-34, and an output portion 35. The middle portion 32-34decomposes itself into: a first taper portion 32; an amplifier portion33; and a second taper portion 34. The portions are successive, but thatdoes not necessarily exclude other intermediate sections, between theportions. However, the preferred embodiments include successions ofportions and only such portions, as indicated above.

The middle portion is embedded in the cladding material 10; it faces thepassive waveguide 20, i.e., a lower surface S1 of the middle portion isvis-à-vis an upper surface S2 of the passive waveguide 20. The terms“lower” or “upper” surfaces imply a relative configuration of the twosurfaces, i.e., the layout could obviously be reversed with the passivewaveguide above the active waveguide. In addition, the two surfaces S1and S2 need not be in contact. As seen in FIGS. 2-5, some material canbe provided between the two waveguides, where the transverse distance istypically between 0 and 5 μm.

The portions 32, 34 are meant to act as couplers, as discussed later.They are tapered in-plane, where each taper 32, 34 widens towards theamplifier portion 33 in a plane that is parallel to the lower surfaceS1. The narrow end of each of the tapers has a cross-sectional area thatis smaller than the cross-sectional area of an opposite section of thepassive waveguide 20 at the level of the narrow end. In other words, thewidth of the first taper portion 32 increases from an end of the inputportion 31 to a first end of the amplifier portion, and the width of thesecond taper portion 34 decreases from a second end of the amplifierportion 33 to an end of the output portion 35 (from left to right inFIG. 1 or 6), such that the innermost width of each of the taperportions 32, 34 is larger than its outermost width. Preferably,amplifier portion 33 is straight, its width corresponds to that of thetapers at their wide end, and the thickness is the same as the thicknessof the tapers 32, 34 and other portions of the active waveguide. Othershapes for the amplifier portion can be contemplated such as a ring,spiral or simply bended waveguides. The input and output portions canfor instance have the same width as the tapers at the level of theirnarrow end.

Whereas the middle portion extends above the passive waveguide 20, theinput and output portions are not necessarily superimposed to thepassive waveguide. Furthermore, the input and output portions need notnecessarily be embedded in the cladding, contrary to the middle portion.In that respect, it should be noted that, notwithstanding the primaryrole played by the middle portion 32-34, the active waveguide 30 extendsbeyond the couplers 32, 34 in the present case, owing to the input andthe output sections 31, 35, which allow for inputting and collectingoptical signal. Notably, the pump signal is meant to be input to theamplifier portion 33 through the active waveguide, via the input portion31.

The gain material is embedded in the active waveguide 30 as a whole andthe input and output sections contain gain material. Gain materials arematerials that can be pumped at a wavelength, which typically is 60 to80% of the signal wavelength or shorter. These are approximate numbers,as usually considered in the art, as there is no sharp boundary defined.As the pump wavelength approaches the signal wavelength, it becomes moredifficult to achieve high extinction ratio for the pump and lowactive-passive coupling loss for the signal simultaneously.

As present inventors have realized, such a design and in particular thetaper orientation allow for adiabatic optical coupling of a signalradiation from the passive waveguide 20 to the active waveguide 30 atthe level of the input portion and then from the active waveguide 30 tothe passive waveguide 20 at the level of the output portion, while notenabling inter-waveguide optical coupling of the pump radiation. As aresult, the present invention enables amplification of the signalradiation and an efficient filtering of the pump radiation, withoutrequiring any additional filter or demultiplexer. Accordingly, a device100 according to embodiments is preferably deprived of any additionalfilter to separate the pump signal from the signal of interest.

Adiabatic optical coupling is known per se. Adiabaticity condition ismet when the optical distribution is defined by the same eigenmode(i.e., supermode) of the coupled waveguide system (e.g., fundamentaleven supermode, fundamental odd supermode) throughout the taper, withminimal scattering to other supermodes or radiation modes. In thepresent case, the supermode profile is transformed along the couplerthrough a taper 32, 34. It overlaps mostly with the passive waveguide 20at the input of the first taper 32 and at the output of the taper 32 itis mainly confined in the active waveguide 30. In this way,inter-waveguide coupling is achieved without any resonance conditions.The situation is reversed at the second taper 34: the supermode istransferred from the active waveguide to the passive waveguide.

The aforementioned scattering loss is never perfectly zero. Adiabaticityis a relative term, as known. A coupler is considered to be adiabaticwhen the loss is below a predefined level, e.g. less than 15%, buttypically less than 10%. At least some of the present embodiments allowfor achieving losses that are less than 5%.

The present invention provides a method of amplification. Referring toFIG. 8, this method includes the following steps. In Step S10, anoptical waveguide amplifier device is provided. Next, at Step S20 andS30, a pump signal is provided to amplifier portion 33 via input portion31. Concomitantly, a signal to be amplified is provided to the passivewaveguide 20. Both signals are input in parallel, i.e., from left toright in FIG. 1 or 6. At Step S40, the signal to be amplified couplesfrom the passive waveguide 20 to the active waveguide 30 at the level ofthe first taper portion 32, which involves adiabatic modetransformation. In Step S50, the pump signal and the signal to beamplified both propagate in the amplifier portion 33 where amplificationtakes place. Last, at Step S60, the amplified signal couples back to thepassive waveguide 20 at the level of the second taper portion 34.

Referring now to FIGS. 1, 6, in various embodiments of the presentinvention, the width of each of the taper portions 32, 34, as taken in aplane parallel to the lower surface S1, decreases non-linearly from theamplifier portion to the narrow end of the tapers 32, 34. As it can berealized, some types of non-linear tapers ensure lower, possibly minimalcoupling losses in a more compact layout/shorter length. This is becausea suitable design allows a smoother transformation of the optical modeensuring minimal scattering to the unwanted modes. Nonlinear taperdesigns can for instance include parabolic segments. Furtherinvestigations on this matter have shown that a parabolic shape is notthe most optimal geometry. Still, it can be regarded as an approximationto the optimal geometry, and at least as a better approximation thanlinear tapers.

In preferred embodiments, each of the taper portions 32, 34 decomposesinto at least two sub-portions, whose respective widths as measured in aplane parallel to the lower surface S1 vary quite differently. In otherwords, the derivative of the width w varies substantially even thoughthe width w of the taper constantly increases along axis x (e.g., afactor of at least 3 is observed between the lowest and highest valuesof the derivative). Formally, each of the taper portions 32, 34decomposes into at least two sub-portions including at least onefast-varying sub-portion 321,323 and at least one slow-varyingsub-portion 322 (see FIG. 9). Preferably, a taper portion decomposesinto three sub-portions 321, 322, 323. Namely, it decomposes into afast-varying sub-portion 321, followed by a slowly-varying sub-portion322, itself followed by a fast-varying sub-portion 323. The averageangle in a fast-varying sub-portion can, for instance, be at least 3times larger than an average angle in a slowly-varying sub-portion. Theaverage angle means an average of tangential angles observed in a givensub-portion, measured in a plane parallel to said lower surface (S1). Atangential angle is the angle between a tangent line to an edge of agiven sub-portion and a longitudinal axis x of the taper portion.

For instance, a “slow” variation (i.e., small angle with respect to thelongitudinal axis of the taper portion) can be provided in the middlesub-portion 322 of a taper portion 32, and the taper angle increasestowards both ends of the taper portions. In the slowest region, thetaper angle can be smaller than 0.1 degrees. On the other hand, theangle can be significantly larger (i.e., larger than 1 degree) in the“faster” regions. FIG. 9 is not to scale for purposes of clarity sinceactual angles are much smaller than the angles depicted.

The various embodiments of the present invention can be realized using asingle non-linear taper section or using multiple taper sections such asa linear section followed by a non-linear section, itself followed by alinear section, etc. Preferred designs of the taper depend on thecoupling efficiency target, geometry and refractive indices of thewaveguides, and the size limitations. Depending on the availablefabrication techniques, it can be more practical to approximate anon-linear taper portion by multiple successive linear sub-portions.

Adiabaticity was explored in the prior art. An analytical formuladescribing tapers having optimal designs was derived in the prior art.Also, the prior art defines adiabaticity criterion and the shortestadiabatic mode transformer in a coupled waveguide system. In particular,the reader is referred to parameters defined by the coupled mode theory.

Referring now to FIG. 6, the passive waveguide 20 can include taperportions 22, 24. Note that in FIG. 6, the cladding 10 is not representedfor clarity. Each of said taper portions 22, 24 face a taper portion 32,34 of the active waveguide 30. However, taper portions 22, 24 havereverse orientation compared to taper portions 32, 34, such that taperportions 22, 24 narrow inwardly in a plane parallel to the upper surfaceS2. This additional feature is believed to enhance the couplingproperties. As a result, the cross-sectional area of the portion of thepassive waveguide 20 that faces the amplifier portion 33 is smaller thanthe cross-sectional area of outermost portions. The passive waveguidecan be interrupted at the level of the amplifier portion since both thesignal to be amplified and the pump signal propagate through the activewaveguide at this stage. In this case, the passive waveguide 20decomposes into two passive waveguide components, where one terminatesat the narrow end of the first taper portion 22 and the other componentterminates at the narrow end of the second taper portion 24 of thepassive waveguide. However, such a limit case may not be optimal. Fromthe fabrication perspective, it may be easier to keep a thin portion ofpassive waveguide facing the amplifier portion 33. This also wouldmitigate the risk of spurious effects such as those caused byfabrication defects at the end of the taper portions 22, 24. Thedimensions of the taper portions 22, 24 are typically similar to thoseof taper portions 32, 34.

Referring now to FIGS. 4, 5, it is of particular advantage to design thelower waveguide 20 as an inverted rib waveguide by including a slab 26and a strip 28 superimposed directly onto the slab. The upper surface S2of the passive waveguide 20 is in that case an upper surface of the slab26 located opposite to the strip 28 with respect to the slab 26. Inother words, the passive waveguide exhibits a T-shape. Both the slab andthe strip are preferably made of the same material. The thickness of theslab 26 is typically smaller than ⅓ of the total thickness of theinverted rib waveguide. The thickness of the slab can vary significantlydepending on the waveguide fabrication technology and the design.

In the present context, using an inverted rib waveguide is advantageousin terms of fabrication process. It allows for obtaining a uniform, flatsurface for a layer of cladding material deposited on top of the slab.This, in turn, allows for depositing or arranging the active waveguide30 above this layer of cladding material. The resulting waveguide 30extends parallel to the passive waveguide. Enhanced coupling propertiesresult in fine.

The distance between the lower surface S1 and the upper surface S2 istypically between 0.0 and 5.0 μm as seen FIGS. 2-5. The length of thetaper portions 22, 24 or 32, 34 are typically between 50 μm and 10 mm,which allows for exceeding adiabaticity limits in practice. The lengthof the amplifier portion 33 can be between 100 μm and 100 cm dependingon the context and desired amplification.

The gain material of the active waveguide can include a dispersion ofgain elements, such as dye molecules, rare-earth ions (e.g., Er3+ ions)or nanocrystals. The latter are preferably designed as quantum dots orquantum rods embedded in a polymer matrix. For instance, the gainmaterial can include a colloidal dispersion of nanocrystals in a core orcore-shell configuration (e.g., nanocrystals including group II-VI,IV-VI, III-V or IV semiconductor materials).

In terms of optical properties, the gain material in the activewaveguide 30 has a higher refractive index than the passive waveguide20. In addition, the refractive index contrast between the passivewaveguide and the cladding material is typically between 0.005 and 0.02.

The original aspects of the embodiments of the present inventiondescribed above reside in the design of active waveguide 30 thatincludes in-plane tapering portions 32, 34, which couple the opticalsignal efficiently, but do not couple the pump signal. The signalcouples to active waveguide 30 at input taper portion 32, propagates ingain material 33, and couples back to passive waveguide 20 at outputtaper portion 34. The pump light remains in the active waveguide suchthat no additional filter is required to separate the pump from thesignal of interest. As explained in the following section, such couplershave an easy design and a high tolerance to design parameters. There areother advantages of the various embodiments of the present invention.For instance, the design of the coupler produces: a substantialconfinement of the optical signal within the active waveguide and asubstantial overlap between the pump and the signal. Also, there isefficient separation of the pump signal from the optical signal at theoutput without using external components, such as filters ordemultiplexers. The vertical twin waveguide approach allows little to noperturbation to the characteristics and the fabrication process of thepassive waveguide. Furthermore, the use of an adiabatic modetransformation instead of directional coupling based on resonantconditions allows for large tolerance to variations of the refractiveindices and dimensions, and positional inaccuracy.

The above embodiments have been succinctly described in reference to theaccompanying drawings and can accommodate a number of variants. Severalcombinations of the above features can be contemplated. Examples aregiven in the next section.

FIG. 1 shows the schematic layout of the amplifier 100. At the input 31of the amplifier, the active waveguide 30 approaches the passivewaveguide 20 (and vice versa). Active and passive waveguides stay inproximity for a minimum length, for inter-waveguide optical coupling. Inthe region of the first taper 32, the width of the active waveguideincreases gradually, in-plane, to couple the signal from the passivewaveguide 20 to the active waveguide 30, and this, by way of adiabaticmode transformation. At the end of the first coupler 32, the signal andthe pump both propagate in the amplifier region 33. This is the regionwhere amplification begins to takes place. At the end of the amplifierportion 33 (which can include spirals, etc.), another coupler 34transfers the signal back to the passive waveguide 20, as explained inthe previous section. The couplers 32, 34 proposed herein favoradiabatic mode conversion.

FIGS. 7A-7F show the refractive indices (FIGS. 7A, D) and mode profiles(7B, C, E, F) at the inner (left-hand column) and outer (right-handcolumn) sides of a tapered section 32. The results of FIG. 7 have beenobtained by optical mode simulation using the film mode matching methodfor the transverse-electric (TE) polarization of radiation. As anexample, the active waveguide composed of colloidal quantum dotsembedded in a Poly(methyl methacrylate) (PMMA) matrix. For the presentexample, a quantum dot volume ratio of 10.0% (volume quantum dot/totalvolume). The refractive index of the quantum dot layer is 2.3, whilePMMA has a refractive index of 1.48. By linear interpolation, it can beassumed that the effective refractive index of the active waveguide tobe 1.56 (2.3·0.1+1.48·0.9). For the core of the passive waveguide, therefractive index used in the simulation is 1.525. The refractive indexof the cladding material used in the simulations is 1.517. Thegeometrical design parameters used in the mode simulations are asfollows. The passive waveguide has a constant width of 8 μm and athickness of 4 μm. The thickness of the cladding between the active andpassive cores is 0.5 μm, and the thickness of the active waveguide coreis 0.75 μm. The width of the active waveguide core is 8 μm at the wideend of the taper and 0.75 μm at the narrow end.

The active waveguide 30 is wide at the inner side of the couplers 32, 34(output of the first taper 32 and input of the second taper 34) andnarrow at their outer side, see e.g., FIGS. 1-3. The active waveguide 30(core) has a higher refractive index than the passive waveguide 20, asseen in FIGS; 7A, D. FIG. 7B depicts a fundamental mode profile at awavelength of 400 nm (pump signal), inputted to the active waveguide 30via the input section 31. FIG. 7C shows the fundamental mode profile atthe wavelength of 1550 nm (signal) at the end of the taper 32 and thebeginning of the taper 34. Therefore, the eigenmode of the coupledwaveguide system (supermode) is confined in the active waveguide 30 atboth pump and signal wavelengths when the active waveguide is wide. Onthe other hand, when the active waveguide is narrow, the supermodeprofiles at the pump and signal wavelengths are considerably different(FIG. 7E, F). The pump light at 400 nm (FIG. 7E) is confined in theactive waveguides, whereas the signal at 1550 nm is essentially confinedin the passive waveguide 20. The signal is affected by tapering muchmore than the pump because of its longer wavelength. If the modetransformation between the mode profile plotted in FIG. 7C and the oneplotted in FIG. 7F is provided adiabatically, the signal is transferredbetween the active waveguide 30 and the passive waveguide 20 with highefficiency, whereas the pump signal remains confined in the activewaveguide 30.

In the example of FIG. 7, the power overlap between the pump signal (400nm) and the signal to be amplified (1550 nm) is ˜45%. The extinctionratio of the pump signal (400 nm) at the end of taper 34 is higher than25 dB. This is the ratio between the pump radiation power in the activewaveguide at the inner side of taper 34 and the residual pump power inthe passive waveguide at the outer side of taper 34. As the pumpwavelength gets longer, the overlap between the pump mode and the signalmode increases in the amplifier portion, whereas the extinction ratio ofthe pump radiation reduces. The pump wavelength can be optimized basedon the design specifications. The coupler loss at the signal wavelength(1550 nm) shall approach 0.01 dB for nearly ideal adiabatic coupling.

Typical lengths of the couplers 32, 34 can for instance be between 50 μmand 10 mm (but more preferably between a few hundred micrometers to afew millimeters). Adiabatic couplers do not have an optimal length. Theloss of adiabatic couplers is insensitive to the length provided thetaper is longer than the adiabaticity limit. If the couplers 32, 34 aredesigned with a safety margin above the adiabaticity limit, the couplerefficiency is insensitive to variations of dimensions, refractive index,etc.

Concerning the dimensions: adiabatic couplers as contemplated herein canhave a few micrometers of tolerance to the misalignment betweenwaveguides. Assume that A is the free-space wavelength of the signalradiation. The cross-sectional dimensions depend on the refractiveindices of the materials. The refractive index contrast between the coreand the cladding of the passive waveguides can have a very broad rangeof values. Suitable values are in the range of 0.005-0.02, as notedearlier. The active waveguide core 30 should have a higher refractiveindex than the passive waveguide core 20 for the disclosed couplers tofunction. The thickness and the width of the passive waveguide core cantypically vary between λ and 6λ, preferred values being approximately3λ-4λ. The passive waveguide shall preferably be a buried channelwaveguide (rectangular core buried in the cladding 10), but it can alsobe an inverted rib waveguide (thin slab with a rectangular protrusion),as discussed earlier.

At the narrow end of a taper 32, 34, the active waveguide core 30 alwayshas a smaller cross-sectional area than the passive waveguide. Thethickness of the active waveguide 30 can have values between 0.2λ and4λ. The simulations reported in FIG. 7 were carried out for a thicknessof ˜0.5λ. Similarly, the width of the active waveguide at the narrow endof the taper can have values between 0.2λ and 4λ. At the wide end of thetaper, the width can be as large as 6λ.

The thickness of the gap between the waveguide cores can vary betweenzero (direct contact between cores 20, 30) and 4λ, i.e., between 0.0 and5.0 μm in practice. The coupling efficiency for the signal reduces asthe gap thickness increases. On the other hand, the extinction ratio ofthe pump at the output increases with the gap thickness. Preferredvalues of the gap thickness are approximately equal to λ.

The length of the amplifier portion 33 (excluding the couplers 32, 34)depends, in practice, on the material gain, confinement factor and thegain target. For example, quantum dots typically have a material gain of˜100 cm−1. Assuming a confinement factor of 50% with a 5% of quantum dotconcentration, a gain of 10 dB is achievable at a length of ˜1 cm. Asanother potential gain material, Er3+ has a significantly lower materialgain, which means that longer waveguides are required to obtain an equalamount of gain. Because of the large variability of the active materialand the confinement factor, the length of the amplifier can be in abroad range, e.g., from 100 μm up to 100 cm, but preferably between afew hundred micrometers and a few centimeters. If necessary, bends orspirals can be integrated into the amplifier to increase the length.

With regard to materials, passive waveguides can be made of low-lossdielectrics such as polymers, silicon nitride, and silicon oxide orsilicon oxynitride. For the active waveguides, nanocrystals can besimply mixed with a polymer. The fabrication process of activewaveguides made of nanocrystal-doped polymers is similar to that ofpassive polymer waveguides.

Among other active materials, colloidal nanocrystals have a very broadabsorption spectrum. Such characteristic differentiates them frommolecules or rare-earth materials, which usually exhibit a relativelynarrow absorption band. Moreover, the emission energy can be easilytuned by quantum confinement effect. Particularly good results have beenobtained with PbS/CdS quantum dots and with CdSe/CdS core/shell quantumdots. Clear amplification has been observed at room temperature andunder ambient condition with typical material gain as high as 200 cm−1.

The active and passive waveguides can be stacked vertically, along withnecessary cladding layers, using known deposition methods, such as spincoating, doctor blading, ink jet printing, and spray coating forpolymer-based waveguides. Waveguides made of different materials, suchas silicon oxynitride and silicon dioxide, can be fabricated using knownmaterial deposition (e.g. chemical vapor deposition) and waveguideformation (e.g. etching) methods. The amplifier 100 disclosed herein canbe fabricated completely using known methods.

While the present invention has been described with reference to alimited number of embodiments, variants and the accompanying drawings,it will be understood by those skilled in the art that various changescan be made and equivalents can be substituted without departing fromthe scope of the present invention. In particular, a feature(device-like or method-like) recited in a given embodiment, variant orshown in a drawing can be combined with or replace another feature inanother embodiment, variant or drawing, without departing from the scopeof the present invention. Various combinations of the features describedin respect of any of the above embodiments or variants can accordinglybe contemplated, that remain within the scope of the appended claims. Inaddition, many minor modifications can be made to adapt a particularsituation or material to the teachings of the present invention withoutdeparting from its scope. Therefore, it is intended that the presentinvention not be limited to the particular embodiments disclosed, butthat the present invention will include all embodiments falling withinthe scope of the appended claims. In addition, many other variants thanexplicitly touched above can be contemplated. For example, othermaterials than those explicitly cited could be involved to obtainsimilar or substantially the same technical effects.

The invention claimed is:
 1. An optically pumpable waveguide amplifierdevice, comprising: a cladding material; a passive optical waveguideembedded in the cladding material that has no optical amplificationfunctionality; and an active optical waveguide having a gain materialwith a higher refractive index than the passive optical waveguide, andwhich successively includes: an input portion, a middle portion, and anoutput portion, wherein the middle portion successively includes: afirst taper portion, an amplifier portion, and a second taper portion;the middle portion embedded in the cladding material, and facing thepassive waveguide, such that a lower surface of the middle portion facesan upper surface of the passive optical waveguide; and each of the taperportions widens towards the amplifier portion, parallel to the lowersurface, such that a narrow end of each of the taper portions have across-sectional area that is smaller than a cross-sectional area of thepassive optical waveguide at the same level of narrow end.
 2. Theoptical waveguide amplifier device of claim 1, wherein a width of eachof the taper portions decreases non-linearly from the amplifier portion,parallel to the lower surface.
 3. The optical waveguide amplifier deviceof claim 2, wherein each of the taper portions decomposes into at leasttwo sub-portions, including at least one slowly-varying sub-portion andat least one fast-varying sub-portion.
 4. The optical waveguideamplifier device of claim 1, wherein the distance between the lowersurface and the upper surface is between 0.0 and 5.0 μm.
 5. The opticalwaveguide amplifier device of claim 1, wherein the passive opticalwaveguide is a rib waveguide, comprising: a slab and a strip, the stripbeing superimposed directly onto the slab, and wherein the upper surfaceof the passive optical waveguide is an upper surface of the slab,opposite to the strip with respect to the slab.
 6. The optical waveguideamplifier device of claim 1, wherein a length of the taper portions isbetween 50 μm and 10 mm.
 7. The optical waveguide amplifier device ofclaim 1, wherein a length of the amplifier portion is between 100 μm and100.00 cm.
 8. The optical waveguide amplifier device of claim 1, whereinthe gain material comprises: a dispersion of dye molecules, rare-earthions or nanocrystals embedded in a polymer.
 9. The optical waveguideamplifier device of claim 8, wherein the gain material comprises: acolloidal dispersion of nanocrystals in a core or core-shellconfiguration, the nanocrystals selected from a group consisting of:group II-VI, IV-VI, III-V or IV semiconductor materials.
 10. The opticalwaveguide amplifier device of claim 1, wherein the gain materialcomprises: a dispersion of Er3+ions.
 11. The optical waveguide amplifierdevice of claim 1, wherein the refractive index contrast between thepassive optical waveguide and the cladding material is between 0.005 and0.02.
 12. The optical waveguide amplifier device of claim 1, wherein thedevice comprises: no filter or demultiplexer to separate a pump signalfrom a signal to be amplified.
 13. The optical waveguide amplifierdevice of claim 1, wherein the passive optical waveguide comprises: twotaper portions that narrow inwardly and parallel to the upper surface ofthe middle portion of the active optical waveguide, and wherein eachtaper portion faces the corresponding first taper portion and secondtaper portion of the middle portion in the reverse orientation.
 14. Theoptical waveguide amplifier device of claim 13, wherein the passiveoptical waveguide decomposes into two passive waveguide components,wherein the first component terminates at the narrow end of the firsttaper portion of the passive optical waveguide and the second componentterminates at a narrow end of the second taper portion of the passiveoptical waveguide.
 15. The optical waveguide amplifier device of claim8, wherein the nanocrystals are quantum dots or quantum rods.
 16. Theoptical waveguide amplifier device of claim 2, wherein each of the taperportions decomposes into three consecutive sub-portions, comprising: afast-varying sub-portion; a slowly-varying sub-portion; and a secondfast-varying sub-portion.